varied and may include the proximal phalanx and the dorsal aspect of the
metacarpal/metatarsal condyles.
Studies on the incidence of MRI diagnoses in the fetlock region are scarce. The most
common MRI diagnoses in the fetlock region are subchondral bone abnormalities of
the distal metacarpus and lesions of the DSLs. Cartilage or osteochondral lesions in
the fetlock joint and injuries of the suspensory ligament branches and proximal
sesamoid bones were less common but also occurred with regular frequency.
Subchondral bone injuries can be diagnosed with low field MRI. Combinations of
injuries are common in the fetlock region, with 63% of horses reportedly suffereing
from multiple MRI abnormalities. In one study, concurrent soft tissue injuries were
present in 65% of horses with subchondral bone injury, most commonly involving
components of the suspensory apparatus.
Subchondral bone lesions in the fetlock are predominantly located in the medial
condyle of the distal part of the MC3/MT3 with a preference for the palmar/plantar
aspect of the condyle. The dorsa l aspect of the condyles may also be affected,
especially in non-racehorses. The dorsoproximal aspect of the proximal phalanx is a
well-recognized site of incomplete dorsal cortical fractures in racehorses. Patterns of
osseous damage have also been recognised in the dorsal aspect of the sagittal ridge
of distal MC4/MT3, and adjacent to the sagittal groove of the proximal aspect of the
proximal phalanx. The latter two sites frequently occur in combination in horses with
a fissure fracture or subchondral and trabecular bone trauma of the sagittal groove of
the proximal phalanx.
The subchondral bone thickness of the distal aspect of the MC3/MT3 varies from
dorsal to palmar and from abaxial to axial, being thinnest axially and thickest in the
middle of each condyle, especially toward the palmar aspect. Subchondral bone
thickness of the distal aspect of the MC3/MT3 is likely to change with the type of
exercise the horse performs. The subchondral bone thickness of the proximal
phalanx increases slightly toward the palmar aspect of each condyle. There is
reasonable symmetry in subchondral bone thickness of both the MC3/MT3 and
proximal phalanx. Abnormal MRI signal in subchondral bone is manifested as diffuse
or focal signal increase in fat suppressed images consistent with bone edema or
bruising, and diffuse T1, PD and T2 signal decrease consistent with trabecular
thickening and osteosclerosis. Focal T1, PD and T2 signal increase is observed in
the presence of trabecular necrosis in osseous cyst-like lesions. Fluid-like signal in
bone appears more commonly in the acute stage of injury while sclerosis reflects
more chronic bone damage with reactive modeling. Focal osteonecrosis may be
visible in the center of an area of sclerosis. When located close to the articular
surface, osteonecrosis may lead to secondary articular cartilage loss from the palmar
surface of the affected condyle of the MC3/MT3. Subchondral bone damage may or
may not be associated with primary or secondary cartilage loss. Some authors
consider the presence of post traumatic subchondral bone oedema a risk factor for
the development of degenerative articular cartilage lesions in human patients.
Selected Bibliography
1.
2.
3.
Biggi M, Dyson S, Murray R. Scintigraphic assessment of the metacarpophalangeal and metatarsophalangeal joint of
horses with joint pain. Vet Radiol Ultrasound 2009;50:536–544.
Blunden A, Dyson S, Murray R, Schramme M. Histological findings in horses with chronic palmar foot pain and agematched control horses Part 1: the navicular bone and related structures. Equine vet J. 38:15-22, 2006
Blunden A, Dyson S, Murray R, Schramme M. Histological findings in horses with chronic palmar foot pain and agematched control horses Part 2: the deep digital flexor tendon. Equine vet J. 38:23-27, 2006
Proceedings
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South
African
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Association
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